Huarazensis has no or if at all only very fine, nearly invisible spines while Huanucoensis usually has some. The Distance between the areoles on Huarazensis should be smaller than on Huanucoensis. The areoles on Huarazensis are more oval/quadrangular while the ones on Huanucoensis are more rounded. The overall shape of Huanucoensis is more rounded, while Huarazensis is straighter. Huanucoensis has very broad ribs and tends to have fewer ribs. But we only know very few types from Huaraz and there definitely are more than what we currently know so take that with a grain of salt. Besides, rib count can vary greatly and while most of the Huarazensis I know ended up having 6-7 ribs, it´s definitely possible to come across Huanus with a similar rib count. This is not a real science and every plant can be different.
Compare those two pics from misplant´s website and you see the differences.
This first pic shows the Huanucoensis. And now the Huarazensis.